1. Field of the Invention
The present invention relates to control of a surface acoustic wave (SAW) convolver element of a monolithic structure wherein a piezoelectric film, an insulating film and a semiconductor are laminated, and particularly, to a convolver controller for improving a start timing by controlling an operation of such an SAW convolver element.
2. Description of the Prior Art
An optimum bias circuit for generating a gate bias voltage (voltage applied across a gate electrode and a ground electrode) which maximizes the convolution efficiency to apply the bias voltage to a convolver element of the type mentioned above has been disclosed by Japanese Patent Public Disclosure No. 52509/1988 (Optimum Bias Circuit for Convolver) and No. 77177/1988 (Optimum Bias Circuit in Surface Acoustic Wave Apparatus).
These optimum bias circuits control a gate voltage V.sub.G in accordance with gate capacitance C.sub.G (capacitance across the gate electrode and the ground electrode) in order to make maximize the convolution efficiency F.sub.T. Such a control method utilizes an approximately constant relationship between a C.sub.G -V.sub.G characteristic and an F.sub.T -V.sub.G characteristics, as will be understood from the convolver element (having an N-type substrate) of the type shown in FIG. 1. In more detail, it has been found that an approximately constant relationship exists between an F.sub.T -V.sub.G curve FVC and a C.sub.G -V.sub.G curve CVC in regard to a relative change in the gate voltage V.sub.G, and that the optimum gate capacitance C.sub.Gop when the maximum convolution efficiency F.sub.Tmax is obtained is substantially constant regardless of any change in gate voltage V.sub.G. Therefore, the optimum bias circuits explained above are structured such that characteristics of a particular convolver element to be used are measured, that the optimum value of gate capacitance C.sub.Gop (or a range of change thereof resulting from an allowable temperature change) for obtaining the maximum convolution efficiency (F.sub.Tmax) in a stabilized operating condition is decides beforehand, that the optimum value V.sub.Gop of gate voltage V.sub.G corresponding to C.sub.Gop (or a range of change in V.sub.Gop resulting from a temperature change) is determined, and a range V.sub.L -V.sub.H in which the gate bias voltage V.sub.B varies is determined so as to include the optimum value V.sub.Gop (or the range of change thereof).
Such optimum bias circuits as explained above are disadvantageous in that when a convolver element is started, as will be seen from a V.sub.B -t curve VTC and a F.sub.T -t curve FTC shown in FIG. 2, the time taken until the maximum convolution efficiency can be obtained, namely the warming-up time T.sub.W, becomes unacceptably long (for example, several minutes or more at a low temperature).
It is because the C.sub.G -V.sub.G characteristic and the F.sub.T -V.sub.G characteristic of a convolver element are shifted (for example: .+-.10 volts) at the time of starting (refer to S2 in FIG. 3) from the range in the balanced condition (refer to S1 in FIG. 3) in regard to the absolute value of the gate voltage thereby causing the optimum gate voltage to be deviated from the range in which a predetermined gate bias voltage V.sub.B is variable.
In FIG. 3, the C.sub.G -V.sub.G and F.sub.T -V.sub.G characteristics in the balanced condition S1 are indicated by solid lines CVC1 and FVC1, while these characteristics in the shifted condition S2 are indicated by chain lines CVC2 and FVC2. Moreover, the optimum gate voltage V.sub.Gop2 in the shifted condition greatly deviates from the range V.sub.L -V.sub.H of V.sub.B. A principal factor resulting in such characteristic shifts is, as is well known, a shift of an amount of electric charge in the piezoelectric film of a convolver element from the amount in the stable condition, and such a shift depends on the level of gate bias voltage previously applied and the length of time which has elapsed from the preceding use.